Database of Low-e Storm Window Energy Performance across U.S. Climate Zones

Transcription

1 PNNL-22864, Rev.2 Database of Low-e Storm Window Energy Performance across U.S. Climate Zones September 2014 TD Culp KA Cort, Project Lead

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3 PNNL-22864, Rev.2 Database of Low-e Storm Window Energy Performance across U.S. Climate Zones TD Culp 1 KA Cort, Project Lead September 2014 Prepared for the U.S. Department of Energy under Contract DE-AC05-76RL Birch Point Consulting, LLC, La Crosse, Wisconsin.

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5 Summary The energy savings and cost effectiveness of installing low-emissivity (low-e) storm windows over existing windows in residential homes was evaluated across a broad range of U.S. climate zones. This work expands upon previous case studies, as well as modeling analysis performed in support of the state of Pennsylvania s efforts to add low-e storm windows to its list of priority Weatherization Assistance Program measures. Calculations of energy savings and cost effectiveness of low-e storm windows were conducted with two software platforms: the National Energy Audit Tool (NEAT) used by weatherization programs and RESFEN software used to compare the annual energy performance of different window options in single-family homes. Both exterior and interior low-e storm windows/panels installed in conjunction with three different primary window types were evaluated in 22 different cities across all eight International Energy Conservation Code climate zones. Both regular low-e glass and solar control low-e glass, which decreases solar heat gain in addition to decreasing heat transfer through the glass, were included in the analysis. The NEAT analysis used 39 model homes, and the RESFEN analysis used 2 model homes. Low-e storm windows were found to always be cost effective when installed over single-pane windows in climate zones 3 through 8, and over double-pane, metal-framed windows in climate zones 4 through 8. The savings-to-investment ratio (SIR) ranged from 1.2 to 3.2 across the different locations analyzed. The average source energy savings ranged from 24 to 36% with a simple payback period of 4.7 to 12.9 years across climate zones 4 through 8. The use of solar-control, low-e storm windows is recommended in climate zone 3, and may also be considered in warmer parts of zone 4 where cooling degree days exceed heating degree days. The use of regular low-e storm windows is recommended in zones 4 through 8. Low-e storm windows also were found to qualify as weatherization cost-effective measures (based on SIR greater than 1) and would therefore be recommended for installation over double-pane wood or vinyl-framed windows in climate zones 6 through 8, and in eastern parts of zone 5 where higher heating fuel costs exist. The SIR ranged from 1.1 to 1.9 across the different locations analyzed. The average source energy savings ranged from 16 to 19% with a simple payback period of 11 to 14 years. All analyses were conducted assuming the model home was heated either with a natural gas furnace or electric heat pump depending on location. A sensitivity analysis was performed to examine the impact on the SIR when the assumed heating equipment, fuel sources, fuel costs, and baseline air leakage of the primary window were changed. When propane heating, electrical-resistance heating, or higher than average heating fuel costs are present, the SIR for installing a low-e storm windows increases. The SIR also is significantly higher for installations over primary windows with higher air leakage than what was assumed in this study (3 cfm/ft 2 for single-pane base windows, 1 cfm/ft 2 for double-pane base windows). In addition, the incremental cost for using low-e glass versus clear glass was found to be cost effective in all climate zones over all window types with an average payback period of 2 to 5 years. This indicates that, when a homeowner chooses to install a storm window or interior window panel for reasons other than just energy savings (e.g., increased comfort, noise reduction, window protection, reduced air leakage, etc.), the use of low-e glass is recommended regardless of location. iii

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7 Acronyms and Abbreviations AFUE DOE HSPF IECC LBNL Low-e Mcf NEAT RECS SEER SHGC SIR annual fuel utilization efficiency U.S. Department of Energy heating seasonal performance factor International Energy Conservation Code Lawrence Berkeley National Laboratory low emissivity 1000 cubic feet National Energy Audit Tool Residential Energy Consumption Survey seasonal energy efficiency ratio solar heat gain coefficient savings-to-investment ratio v

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9 Contents Summary... iii Acronyms and Abbreviations... v 1.0 Introduction and Background NEAT Analysis of Low-e Storm Windows NEAT Analysis and Methodology NEAT Results RESFEN Analysis of Low-e Storm Windows RESFEN Analysis Background and Methodology RESFEN Results Conclusions and Recommendations Based on the NEAT and RESFEN Results References Appendix A NEAT Calculation Results for Each City... A.1 Appendix B RESFEN 6 Modeling Assumptions... B.1 Appendix C RESFEN Calculation Results... C.1 vii

10 Figures 2.1 IECC Climate Zone Map Effect of Natural Gas Price on the SIR of Low-e Storm Windows in Chicago Effect of Existing Window Air Tightness on the SIR of Low-e Storm Windows in Pittsburgh, Pennsylvania Overall Recommended Regions for the Use of Low-e Storm Windows Installed Over Single-Pane Windows and Double-Pane Metal-Framed Windows and the Location of Cities Included in this Analysis Overall Recommended Regions for Use of Low-e Storm Windows Installed Over Double- Pane, Wood/Vinyl-Framed Windows Tables 2.1 Average SIR for Low-e Storm Windows Calculated by NEAT Percent Site Energy Savings, SIR, and Simple Payback for Low-e Storm Windows in a Representative Wood-Framed Home as Calculated by NEAT Properties for Storm Panels over Wood Base Windows Properties of Storm Panels over Metal Base Windows HVAC Source Energy Savings for Low-e Storm Windows and Panels as Calculated by RESFEN Smaller Older 1-Story Home HVAC Energy Cost Savings of Exterior Low e Storm Windows over Single Pane Wood Windows as Calculated by RESFEN Smaller Older 1 Story Home HVAC Source Energy Savings for Low-e Storm Windows and Panels as Calculated by RESFEN Larger, Newer, Two-Story Home HVAC Energy Cost Savings of Exterior Low-e Storm Windows over Single-Pane Wood Windows as Calculated by RESFEN Larger, Newer, Two-Story Home Overall Average Savings and Payback Period by Climate Zone. Results in climate zones 1 3 are for solar control low e Effect of Different Mounting Methods for Installing Exterior Storm Windows over Metal- Framed, Double-Pane Windows in Boston, Massachusetts viii

11 1.0 Introduction and Background This report describes work conducted in support of the Emerging Technologies Low-e Storm Windows Task 5.3: Create a Database of U.S. Climate-Based Analysis for Low-e Storm Windows. The scope of the overall effort is to develop a database of energy savings and cost effectiveness of lowemissivity (low-e) storm windows/panels in residential homes across a broad range of U.S. climates. The database expands upon previous calculations for Pennsylvania s Weatherization Assistance Program and storm window case studies to develop regionally based savings information for additional locations and climates. This report and calculations will be made publicly available through the Building America Solution Center and other outreach activities. This research can provide U.S. Department of Energy (DOE), national and regional utilities, and state weatherization programs with important data showing the potential energy savings low-e storm windows can contribute to energy retrofits in existing buildings. A series of laboratory tests have proven that standard low-e storm windows save energy at the component level. The performance improvements have been validated with field tests and case studies supported by DOE s Emerging Technologies team. The combination of results from these laboratory and field tests, as well as the data collected as part of these previous efforts, helped inform the National Energy Audit Tool (NEAT) and RESFEN modeling assumptions used in this study. These case studies have been summarized in the report entitled Task Plan in Support of Emerging Technology Task ET-WIN- PNNL-FY13-01_5.1: Create a Database of U.S. Climate-Based Analysis for Low-e Storm Windows, which was completed in an earlier phase of this overall modeling task (Hefty et. al. 2013). In addition to the field tests and case studies, prior calculations of potential energy savings included a DOE-funded joint effort 1 for the state of Pennsylvania using NEAT to support adding low-e storm windows to the state s weatherization measure priority list, and previous calculations performed by Birch Point Consulting under DOE Office of Energy Efficiency and Renewable Energy award #DE-EE using RESFEN software to estimate savings in different cities. Developed by Oak Ridge National Laboratory for DOE s Office of Weatherization and Intergovernmental Programs, NEAT is the primary approved software designated for state weatherization programs. RESFEN is a DOE-based building simulation program developed by Lawrence Berkeley National Laboratory (LBNL) that was specifically designed to compare the annual energy performance of different fenestration options in single-family homes, and it is used as the basis of the ENERGY STAR program for windows, doors, and skylights. It is valuable to have calculations from both platforms to address the different audiences that will use the data (utilities, weatherization programs, federal energy-efficiency programs, consumers, etc.). Gaps in both existing data sets were identified, and additional NEAT and RESFEN analyses were proposed, specifically adding cities to cover additional U.S. climate zones, and including data for low-e storm windows installed over additional primary window types (single or double pane, wood or metal frame, etc.) The purpose of this database effort and documentation report is to fill in these gaps by modeling low-e storm window performance over a broad range of climate zones with a variety of baseline model home characteristics. This report provides a summary of the final results of the NEAT and RESFEN calculations, including general observations and recommendations regarding the use of low-e storm windows and panels. 1 Joint effort between DOE s BTP and DOE s Office of Weatherization and Intergovernmental Programs (OWIP), Pennsylvania s Department of Community and Economic Development, LBNL, Energetics, and Birch Point Consulting. 1

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13 2.0 NEAT Analysis of Low-e Storm Windows During 2010 and 2011, an analysis was conducted to help the state of Pennsylvania determine the energy savings potential of adding the installation of low-e storm windows to its weatherization measure priority list. This was a joint effort of DOE, the Pennsylvania Department of Community and Economic Development, Energetics, LBNL, and Birch Point Consulting. The prior NEAT analysis included four cities across Pennsylvania, 37 model home types, and two primary window types (single-pane wood frame, and double-pane metal frame). The 37 model homes were selected from a data set, prepared by Dalhoff Associates with the intention of covering a wide range of older existing housing commonly encountered in weatherization programs. Different insulation scenarios were included for the following home types: detached masonry, semi-detached masonry, rowhouse masonry, detached wood frame (e.g., slab, crawl space, unconditioned basement, conditioned basement, etc.), cape-cod wood frame, semi-detached wood frame, row-house wood frame, and exposed floor (Dalhoff 2010). The primary result of the NEAT analysis that examined a particular weatherization measure comes in the form of savings-to-investment ratio (SIR), because a SIR greater than 1 is required to qualify as a weatherization measure using state and federal funding. NEAT also can be used to provide estimates of site energy savings, rankings relative to other weatherization measures, and the maximum cost that a weatherization measure could be while still producing a SIR greater than 1. Simple payback also can be calculated; although, in the case of storm windows, only part of the air-infiltration savings is captured in the storm window listing within the NEAT 1 calculation results (provided in Appendix A). Therefore, either a portion of the overall home air-infiltration savings also needs to be credited to storm windows in the calculation, or the payback period will be somewhat overstated. The analysis for Pennsylvania showed that low-e storm windows would qualify as a cost-effective weatherization measure. The SIR values ranged from 1.4 to 2.2 when low-e storm windows were used over single-pane, wood-framed windows and ranged from 1.3 to 2.1 when they were used over doublepane, metal-framed windows. As a result, the state of Pennsylvania added low-e storm windows to its weatherization measure selection priority list for single-family homes in 2010 (Zalis et al. 2010; Krigger 2011). 2.1 NEAT Analysis and Methodology The NEAT analysis presented in this report expands on the previous weatherization work to cover more locations in different states and climates. As outlined below, the same general methodology used in for Pennsylvania was used for this expanded coverage. 1 NEAT simulates heating, cooling, and air infiltration reductions separately and attributes whole home reductions in air infiltration to a number of different air-sealing weatherization measures. Based on case studies summarized in Hefty et. al. (2013), whole home air leakage can be reduced by an average of 10 to 15% just by adding storm windows, and when combined with other weatherization measures, the whole home leakage is reduced by 20 to 40%. Therefore, assigning an estimate of one-third of air-infiltration reductions (which was assumed in this study) to be attributed to storm windows most likely underestimates the full infiltration benefits of storm windows and would be considered a conservative attribution. 3

14 NEAT calculations were run for a total of 20 cities across seven International Energy Conservation Code (IECC) climate zones (no weather files were available in NEAT format for climate zone 8, which is only found in northern Alaska). Figure 2.1 is a map of IECC climate zones and the locations of cities included in this analysis. The cities are listed out in Table 2.1 and Table 2.2. Low-e storm windows were evaluated based on installations of three different primary window types (i.e., single-pane wood or vinyl frame, double-pane wood or vinyl frame, and double-pane metal frame, all with clear glass). This adds clear double-pane wood and double-pane vinyl windows to the windows addressed in the Pennsylvania NEAT analysis. Single-pane metal windows were not included, but because the SIR will always be higher, they will be qualified as weatherization measures where single-pane wood/vinyl windows or double-pane metal windows are used. This is because the single-pane, metal-framed window will have the worst U-factor (i.e., heat transfer coefficient) of all the primary window types, and therefore, the relative improvement in U-factor and energy performance from adding a low-e storm window will be even higher than with the other primary window types. Figure 2.1. IECC Climate Zone Map The method used in this analysis was different from the Pennsylvania NEAT analysis as follows: The latest NEAT version (i.e., released in February 2012) was used. This version includes updated fuel escalation rates and updated prices for other weatherization measures. Two additional homes were added to the data set that was used in the Pennsylvania NEAT analysis (Dalhoff 2010) to capture a broader range of insulation levels. This resulted in total of 39 model homes, where the two additional homes are assumed to have insulation throughout the 4

15 home (R11 walls, R19 attic, R11 floor) and one is characterized as a wood-frame, vented crawl space home, while the other is a masonry home. For the set of homes used in the Pennsylvania NEAT analysis (Dalhoff 2010), only one-quarter of the floor space was assumed to be air conditioned using less efficient room/window airconditioner units, and no air conditioning was used in the rest of the home. These characteristics are typical of weatherization homes in Pennsylvania and many other locations, but data from the DOE Residential Energy Consumption Survey (RECS) show that it is more common to have central air conditioning in the Midwest, South-Central, and Southern regions (DOE-EIA 2009). Therefore, the homes were modified to include a central air conditioner (SEER 8) in climate zones 1 through 3 as well as in the warmer zone 4 locations where cooling degree days exceed heating degree days (i.e., Raleigh, North Carolina, Washington, D.C.). RECS data also show that heat pumps are more common than natural gas furnaces in southern locations, so the homes were modified to use a heat pump with 6.8 HSPF in climate zones 1 and 2 and certain zone 3 locations rather than an 80% AFUE natural gas furnace (DOE-EIA 2009). The natural gas and electricity prices used were based on location using 2012 state average prices taken from the DOE Energy Information Administration Natural Gas Monthly and Electric Power Monthly reports (DOE-EIA 2013). The assumed storm window properties are described below: Low-e Glass. Storm window properties were based on the characteristics of commercially available low-e storm windows. Standard low-e (0.157 emissivity, 0.75 center-of-glass solar heat gain coefficient [SHGC]) was evaluated in all zones. Solar-control low-e (0.166 emissivity, 0.54 center-of-glass SHGC) was evaluated in climate zones 1 through 3 and certain warmer locations in zone 4 where cooling degree days exceed heating degree days (Raleigh, North Carolina, and Washington D.C.). Product Cost. Product and installation costs were primarily based on analysis performed as part of the Pennsylvania NEAT study (Dalhoff 2010). For this analysis, material costs were assumed to be $7.85/ft 2 of window area, plus an average of $30 per window was assumed for installation expenses. Do-it-yourself installations (which account for 80% of installations) cost only ~$2 per window, whereas contractor installation can cost $60 per window but will vary depending on location. A weighted average cost of $14 per window could have been used based on this information (the Pennsylvania analysis assumed $15 per window installation costs), but because of the high variability, a cost of $30 was used to be conservative. If the lower installation cost is thought to be more applicable (e.g., for a utility program in which consumers will primarily do self-installation), the SIR results will further increase. Lifetime. Twenty years for this study. Fifteen years was used initially in the Pennsylvania analysis, but based on updated information including manufacturer warranties, 20 years is more accurate (Cort 2013). In addition to the main calculations, some additional runs were made to explore the impact of natural gas fuel pricing and different primary window air leakage levels. Overall, approximately 4240 model runs were performed. 5

16 2.2 NEAT Results The average SIR for using low-e storm windows installed over different primary window types in different cities/climate zones is shown in Table 2.1. NEAT SIR output is only provided for cities with SIRs greater than 1. Each value is the average SIR across the 39 different homes for that particular location and window type. The detailed SIR results for each city and home type are provided in Appendix A. The site energy percentage savings and simple payback period for a representative home are shown in Sensitivity: A sensitivity analysis was performed based on independent changes in single variables. In general, the largest variables affecting whether low-e storm windows are cost effective are location (climate), heating fuel cost, existing window tightness, and storm window cost. Factors that have less effect include home type, window area, and orientation. Heating fuel price has a large impact on SIR as much as location/climate, and more than home type. This is particularly noticeable in the Midwest and Rocky Mountains regions where the cost of natural gas has decreased in recent years and is much cheaper than in the Northeast and Southeast. State average natural gas prices vary significantly from approximately $8 to $16/Mcf (DOE-EIA 2013). As an example, Figure 2.2 shows the SIR as a function of natural gas price for low-e storm windows in one home type in Chicago. At the current low cost of $8.22/Mcf, the SIR is 1.4, but it increases by approximately 0.3 for every $2/Mcf increase in price (with a corresponding decrease in payback period). Table 2.2, although simple payback should be viewed with a cautionary note. In the case of storm windows, only part of the air-infiltration savings are captured under the storm window listing within the NEAT results, so either a portion of the overall home air-infiltration savings needs to also be credited to storm windows in the calculation, or the payback period will be somewhat overstated. 1 An estimate of the payback including the additional air-infiltration savings is also included. The following general points and trends were observed: In climate zones 3 through 8, low-e storm windows are always cost effective (SIR >1) when installed over single-pane windows and double-pane, metal-framed windows. The SIR values range from 1.2 to 3.2 across the different locations analyzed. Low-e storm windows also can be cost effective when installed over double-pane wood or vinylframed windows in many locations of climate zones 3 through 8, but SIR values depends on the climate, local fuel cost, and storm window glass type. The SIR values range from 1.1 to 1.9 across the different locations analyzed. Under the following conditions, the SIR increases: In colder climates By using solar-control, low-e glass in warmer climates (zone 3 as well as warmer cities in zone 4 where cooling degree days exceed heating degree days) In locations with higher heating fuel costs Over existing windows that exhibit the highest air leakage. 1 More detail on the structure and content of the NEAT can be found at the Weatherization Technical Assistance Center ( and in related manuals (Gettings 2006). 6

17 In climate zones 1 and 2, storm windows with solar-control, low-e glass can be cost effective, although pragmatically, other low-cost, solar-control measures like solar screens and films will have higher SIR values. The analysis was performed with either a natural-gas furnace or electrical heat pump depending on location. For homes using propane or electrical resistance heating, the SIR of low-e storm windows will be even higher because the effective heating fuel cost and savings from using low-e storm windows will be higher. The reverse is also true; households with more efficient heating equipment and low heating fuel costs would likely have a relatively lower SIR resulting from storm window installations. Climate Zone Table 2.1. Average SIR for Low-e Storm Windows Calculated by NEAT City, State Average SIR of Low-e Storm Windows Used Over Different Primary Window Types Standard Low-e Glass Solar Control Low-e Glass Single-Pane, Wood- or Vinyl- Framed Window Double- Pane, Metal- Framed Window Double-Pane, Wood- or Vinyl- Framed Window Single- Pane, Wood- or Vinyl- Framed Window Double- Pane, Metal- Framed Window Double-Pane, Wood or Vinyl- Framed Window 7 Duluth, MN A Burlington, VT A Minneapolis, MN A Boston, MA A Rochester, NY A Pittsburgh, PA A Chicago, IL B Denver, CO B Boise, ID A New York, NY A Washington, DC A Raleigh, NC A Kansas City, MO 1.5 (1.7) (a) 1.4 (1.7) (a) 1.0 (1.2) (a) 4 C Seattle, WA A Atlanta, GA A Dallas, TX A Jacksonville, FL A Houston, TX B Phoenix, AZ A Miami, FL = SIR <1 under the assumed fuel and product costs. Gray cells not evaluated. (a) Second value for Kansas City shows whole home air conditioned versus one-quarter of floor area air conditioned. Sensitivity: A sensitivity analysis was performed based on independent changes in single variables. In general, the largest variables affecting whether low-e storm windows are cost effective are location 7

18 (climate), heating fuel cost, existing window tightness, and storm window cost. Factors that have less effect include home type, window area, and orientation. Heating fuel price has a large impact on SIR as much as location/climate, and more than home type. This is particularly noticeable in the Midwest and Rocky Mountains regions where the cost of natural gas has decreased in recent years and is much cheaper than in the Northeast and Southeast. State average natural gas prices vary significantly from approximately $8 to $16/Mcf (DOE-EIA 2013). As an example, Figure 2.2 shows the SIR as a function of natural gas price for low-e storm windows in one home type in Chicago. At the current low cost of $8.22/Mcf, the SIR is 1.4, but it increases by approximately 0.3 for every $2/Mcf increase in price (with a corresponding decrease in payback period). Table 2.2. Percent Site Energy Savings, SIR, and Simple Payback for Low-e Storm Windows in a Representative Wood-Framed Home (Home code WFVC6 1 ) as Calculated by NEAT Climate Zone Location Results in WFVC6 Home from Adding Low-e Storm Windows (wood-framed home, vented crawlspace, R11 walls, R19 attic, R11 floor) Standard Low-e Glass Over Single-Pane Wood/Vinyl Window % Site Energy Savings SIR Simple Payback in Years (a) Solar Control Low-e Glass Over Single-Pane Wood/Vinyl Window % Site Energy Savings SIR Simple Payback in Years (a) 7 Duluth, MN 19% (5.7) 6 A Burlington, VT 19% (3.8) 6 A Minneapolis, MN 19% (7.3) 5 A Boston, MA 21% (5.2) 5 A Rochester, NY 20% (4.4) 5 A Pittsburgh, PA 19% (6.6) 5 A Chicago, IL 20% (8.9) 5 B Denver, CO 18% (9.8) 5 B Boise, ID 18% (9.2) 4 A New York, NY 21% (5.5) 4 A Washington, DC 18% (7.8) 16% (7.4) 4 A Raleigh, NC 16% (10.6) 16% (9.9) 4 A Kansas City, MO 18% (7.1) 4 C Seattle, WA 22% (8.4) 3 A Atlanta, GA 16% (9.7) 16% (9.0) 3 A Dallas, TX 13% (8.1) 16% (6.9) 2 A Jacksonville, FL % (7.7) 2 A Houston, TX % (7.3) 2 B Phoenix, AZ A Miami, FL % (7.0) Payback will be shorter for leakier primary windows, higher fuel costs, or propane and electrical resistance heating. -- = SIR <1 under the assumed fuel and product costs Gray cells not evaluated. (a) Second payback value in parentheses is if one-third of the air-infiltration cost savings is assigned to storm windows. This 1 WFVCS6 is the code for a wood-framed home with a vented crawl space. 8

19 is just an estimate to help bracket the payback. Figure 2.2. Effect of Natural Gas Price on the SIR of Low-e Storm Windows in Chicago The air tightness of the existing window also has a large impact on energy savings and whether the measure is cost effective. The leakier the existing window, the greater the SIR of adding lowe storm windows. NEAT defines five levels of air tightness for the existing window (very tight, tight, medium, loose, and very loose) 1 and provides descriptions to help the auditor assign the appropriate level (see Figure 2.3). The NEAT analysis was performed with a medium tightness, which should be conservative for most existing homes based on the NEAT definition. However, the SIR will increase (and the payback period will decrease) for existing windows that are leakier. As an example, Figure 2.3 shows the SIR for low-e storm windows as a function of existing window air tightness in one home type in Pittsburgh, Pennsylvania. With loose existing windows, which would not be uncommon in older homes, the SIR can increase by 0.8, and the payback period can decrease by over 2 years. 1 Although specific measurable parameters (e.g., cfm/s.f.) are not associated with NEAT categories for air tightness, qualitative defintions are provided for auditing purposes. For example, medium tightness is described as typical of older windows found in older homes. Deteriorating weatherstripping would be present, but no visible gaps. More information can be found in Window Leakiness Guidelines (ORNL 2009). 9

20 Figure 2.3. Effect of Existing Window Air Tightness on the SIR of Low-e Storm Windows in Pittsburgh, Pennsylvania The SIR of low-e storm windows does not heavily depend on home type. The percent energy savings will vary more based on the specific home, but the SIR is less sensitive. As seen in Appendix A, the SIR typically varies by only ±0.2 across all 39 home types for a given window type and city. Although counter-intuitive, window area does not affect the SIR significantly. Window area obviously affects the overall load, energy savings, and storm window cost, but because the cost and energy savings scale similarly with square footage, the SIR does not change much. When the window area was doubled in the WFVC6 home in Pittsburgh, the energy savings increased from 19 to 33% of the total home site energy use, but the SIR only changed from 1.9 to 1.8. The overall cost effectiveness does not vary dramatically with orientation. For the WFVC6 home in Pittsburgh, the SIR remained at 1.9 whether the home was oriented north/south (67% of glazing on the north and south) or oriented east/west. The cost effectiveness of windows on individual sides does vary because of the different beneficial or detrimental role of solar gain on the south versus west/east sides, but the overall SIR does not vary dramatically. (For example, the SIR for solar control low-e storm windows will be higher on the west and east sides than on the south side.) Overall conclusions and recommendations based on both the NEAT and RESFEN analysis results are provided in Chapter

21 3.0 RESFEN Analysis of Low-e Storm Windows RESFEN is the standard software program used for calcuting the impact of windows on heating and cooling costs for new and existing residential homes. RESFEN allows you to enter housing characteristics (including window characteristics) and location information to represent the house type and climate zone of interest. RESFEN is frequently employed by utility energy-efficiency programs and ENERGY STAR to determine whether or not the residential fenestration performance of a given product is sufficient to meet program requirements. 3.1 RESFEN Analysis Background and Methodology Birch Point Consulting performed previous unpublished RESFEN calculations in 2012 for two home types in 30 different cities across seven IECC climate zones. The two home types included an older, smaller, one-story 1800-ft 2 home, and a larger, newer, two-story 2300-ft 2 home. Exterior and interior low-e storm windows installed over one primary window type (single-pane, wood-frame window) were evaluated. In addition, some extra calculations were added in three cities for interior and exterior low-e storm windows installed over both single- and double-pane, wood- and metal-frame windows, and compared to ENERGY STAR replacement windows. Because RESFEN is commonly employed as an evaluation tool for energy-efficiency programs, this study expands and refines previous RESFEN calculations to examine low-e storm window performance using multiple house types and assuming a broad range of climate zones to help inform energy-efficiency programs of low-e storm window performance throughout the U.S. The primary output from the RESFEN analysis includes the percentage of whole house energy savings derived from installation of low-e storm windows, along with heating and cooling energy and cost savings. The payback period also can be calculated, but again, the simple payback should be viewed with a cautionary note, as it is not clear that the full effect of reduced air infiltration attributable to storm windows is accounted for in RESFEN. 1 As part of this study, the RESFEN modeling expands upon previous calculations to cover all U.S. climate zones, add more primary window configurations, use updated U-factor and SHGC values for different storm window and primary window combinations, and update the calculation methodology to be consistent with that used in the ENERGY STAR program for windows, doors, and skylights. The RESFEN analysis was conducted as outlined below. RESFEN calculations were run for the same cities as in the NEAT analysis shown in Figure 2.1, plus two additional cities in climate zones 7 and 8 in Alaska (Anchorage and Fairbanks). This is a total of 22 cities across all eight IECC climate zones. RESFEN version was used, whereas the prior analysis used RESFEN version 5. RESFEN 6 is the version used to help establish criteria for the ENERGY STAR program for windows, doors, and skylights. Several assumptions for the baseline building were updated as shown in Appendix B, although the general conclusions for relative window comparisons do not change substantially from version 5. 1 As with the NEAT analysis, it appears that default assumptions built into RESFEN may under estimate the infiltration reductions attributable to the installation of storm windows over primary windows. 11

22 Two homes were modeled: a smaller, older, one-story 1700 ft 2 home, and a larger, newer, two-story 2800 ft 2 home. These two homes also are used in the analysis for the ENERGY STAR program. The older home has minimal insulation, and the newer home is insulated to the 2006 IECC requirements. Details are shown in Appendix B for the one-story, wood-framed home representative of existing construction and the two-story, wood-framed home representative of new construction. Natural-gas heating was used in most cities, except a heat pump was used in climate zones 1 and 2 and certain zone 3 locations where RECS data show that heat pumps are more dominant (DOE-EIA 2009). Central air conditioning cooling was included in all locations. The natural-gas and electricity prices used were based on 2012 state average prices taken from the DOE Energy Information Administration Natural Gas Monthly and Electric Power Monthly reports (DOE-EIA 2013). The window area was assumed to be 15% of equally distributed floor area, which is the same as the analysis for the ENERGY STAR program. This is 255 ft 2 for the smaller older one-story home, and 420 ft 2 for the larger newer two-story home, or approximately 17 and 28 windows, respectively. Both exterior and interior low-e storm windows and panels were evaluated when installed over three different primary window types (single-pane wood frame, double-pane wood frame, and double-pane metal frame, all with clear glass). Single-pane metal windows were not included, but will be qualified as weatherization measures for cases in which single-pane wood/vinyl windows or doublepane metal windows are used, because the energy savings and cost effectiveness will always be higher. This is because the single-pane, metal-framed window will have the worst U-factor (heat transfer coefficient) of all the primary window types; therefore, the relative improvement in U-factor and energy performance from adding a low-e storm window will be even higher than with the other primary window types. Standard low-e glass was modeled in all locations. In addition, solar-control low-e glass also was modeled in southern locations (climate zones 1 through 3, and certain warmer zone 4 locations where cooling degree days exceed heating degree days). The SHGC of the solar control low-e glass was 27% lower than the standard low-e glass. Solar-control low-e storm windows are designed for exterior application, so interior panels with solar-control low-e windows were not modeled. Clear glass storm windows also were modeled for comparison. To determine the U-factor and SHGC properties for use in the RESFEN analysis, an independent National Fenestration Rating Council-accredited simulation testing laboratory (Architectural Testing, Inc.) conducted detailed calculations of exterior and interior low-e panels installed over various primary windows using THERM and WINDOW software from LBNL (part of DOE Project DE- EE ). The resulting U-factor, SHGC, and Vermont (VT) numbers are shown in Table 3.1 and Table 3.2. This includes many combinations and window types, so the subset in bold text within the tables indicates that these window types were used with the RESFEN analysis. The most accurate method for modeling windows in RESFEN is to import the detailed solar angle dependent properties from WINDOW, rather than just inputting the simple U-factor and SHGC numbers. However, RESFEN and its underlying DOE2.1E software can only use generic frames rather than the detailed frame mounting modeled by Architectural Testing Inc. Therefore, after consultation with the RESFEN developers at LBNL, the window and solar angle properties were imported by creating windows with generic frames and adjusting the frame properties until the whole window U-factor and SHGC matched the same values shown in Table 3.1 and Table

23 Air leakage was modeled as 3 cfm/ft 2 for single-pane base windows, 1 cfm/ft 2 for double-pane base windows, 0.3 cfm/ft 2 with exterior storm windows installed, and 0.1 cfm/ft 2 with interior panels installed (Drumheller 2007). 1 For simple payback period calculations, the same product cost data was used as in the NEAT analysis. This is a product cost of $7.85/ ft 2 of window area, plus $30 per window for installation. To calculate the incremental payback period of low-e glass versus clear glass, a product cost of $6.85/ ft 2 of window area was used for clear glass storm windows and panels, or 13% lower than the low-e storm window. The installation cost was the same (Cort 2013). Overall, approximately 950 simulations were performed. Table 3.1. Properties for Storm Panels over Wood Base Windows (window types in bold text were used as inputs in RESFEN modeling) Base Window Storm Type U-Factor SHGC VT Wood Double Hung, Single Glazed Wood Double Hung, Double Glazed Wood Fixed, Single Glazed Wood Fixed, Double Glazed Clear, Exterior Clear, Interior Low-e, Exterior Low-e, Interior Clear, Exterior Clear, Interior Low-e, Exterior Low-e, Interior Clear, Exterior Clear, Interior Low-e, Exterior Low-e, Interior Clear, Exterior Clear, Interior Low-e, Exterior Low-e, Interior Specific air-tightness parameter assumptions are based on previous case studies, which measured cfm/s.f. results documented in Drumheller

24 Table 3.2. Properties of Storm Panels over Metal Base Windows (window types in bold text were used as inputs in RESFEN modeling) Base Window Storm Type U-Factor SHGC VT Aluminum Double Hung, Single Glazed Worst case mounting Clear, Exterior Thermally broken mounting (recommended) Clear, Exterior Clear, Interior Worst case mounting Low-e, Exterior Thermally broken mounting (recommended) Low-e, Exterior Low-e, Interior Aluminum Double Hung, Double Glazed Worst case mounting Clear, Exterior Thermally broken mounting (recommended) Clear, Exterior Clear, Interior Worst case mounting Low-e, Exterior Thermally broken mounting (recommended) Low-e, Exterior Low-e, Interior Aluminum Fixed, Single Glazed Worst case mounting Clear, Exterior Thermally broken mounting (recommended) Clear, Exterior Clear, Interior Worst case mounting Low-e, Exterior Thermally broken mounting (recommended) Low-e, Exterior Low-e, Interior Aluminum Fixed, Double Glazed Worst case mounting Clear, Exterior Thermally broken mounting (recommended) Clear, Exterior Clear, Interior Worst case mounting Low-e, Exterior Thermally broken mounting (recommended) Low-e, Exterior Low-e, Interior RESFEN Results Table 3.3 through Table 3.6 show the energy savings results for using low-e storm windows installed over different primary window types in different cities/climate zones. Table 3.3 and Table 3.5 show the source energy savings for using low-e storm windows in the smaller, older, one-story home and the larger, newer, two-story home, respectively. Similarly, for the smaller, older, one-story home and the larger, newer, two-story home, Table 3.4 and Table 3.6 show the energy cost savings, simple payback period for the total storm window, and incremental simple payback period for using a low-e storm window versus a clear storm window. The detailed results for each city and home type are included in Appendix C. Table 3.7 provides a summary of the overall average source energy savings and simple payback periods for each climate zone. The results are averaged over both home types, all cities modeled in each climate zone, and both interior and exterior low-e panels. 14

25 Table 3.3. HVAC Source Energy Savings for Low-e Storm Windows and Panels as Calculated by RESFEN Smaller Older 1-Story Home Climate Zone City, State HVAC Source Energy Savings of Low-e Storm Windows Used Over Different Primary Window Types in Smaller, Older, One-Story Home Single-Pane, Wood- Framed Window Standard Low-e Glass Double- Pane, Metal- Framed Window Double-Pane, Wood- Framed Window 8 Fairbanks, AK 18 20% 12 13% 8 10% 7 Anchorage, AK 23 25% 15 17% 10 12% 7 Duluth, MN 24 26% 16 17% 10 13% Single- Pane, Wood- Framed Window Solar-Control, Low-e Glass Double- Pane, Metal- Framed Window Double-Pane, Wood- Framed Window 6 A Burlington, VT 25 28% 17 18% 11 14% 6 A Minneapolis, MN 25 28% 16 18% 12 13% 5 A Boston, MA 28 30% 18 20% 12 15% 5 A Rochester, NY 24 26% 16 17% 11 13% 5 A Pittsburgh, PA 24 26% 16 17% 11 13% 5 A Chicago, IL 25 27% 17 18% 12 13% 5 B Denver, CO 24 27% 17 18% 11 13% 5 B Boise, ID 25 27% 17 18% 12 13% 4 A New York, NY 25 27% 16 18% 11 13% 4 A Washington, DC 24 26% 16 17% 11 13% 22% 14% 10% 4 A Raleigh, NC 19 21% 13 14% 9 10% 19% 12% 8% 4 A Kansas City, MO 26 28% 17 19% 13 14% 26% 16% 12% 4 C Seattle, WA 29 32% 20 22% 14 16% 3 A Atlanta, GA 24 26% 16 17% 12 13% 25% 17% 13% 3 A Dallas, TX: Furnace Heat pump 25% 21 22% 16% 13 14% 13% 12% 26% 24% 18% 16% 2 A Jacksonville, FL 18 17% 11% 11 9% 23% 16% 15% 2 A Houston, TX: Furnace Heat pump 21% 19 18% 13% 11% 12 11% 11 10% 26% 24% 18% 16% 15% 15% 17% 16% 2 B Phoenix, AZ 18 17% 12% 11 10% 23% 17% 16% 1 A Miami, FL 13 11% 7 6% 10 7% 21% 15% 17% Notes: Range of savings indicates exterior and interior low-e panels (first and second numbers, respectively). Solar-control, low-e panels were modeled for exterior application only. Installation over metal-framed window assumes thermally broken mounting. Gray cells not evaluated. 15

26 Table 3.4. HVAC Energy Cost Savings of Exterior Low e Storm Windows over Single Pane Wood Windows as Calculated by RESFEN Smaller Older 1 Story Home Climate Zone Location HVAC Energy Cost Savings of Exterior Low e Storm Windows in Smaller Older 1 Story home Standard Low e Glass Over Single Pane Wood Window Energy Cost Savings Simple Payback for Total Product (yrs) Simple Payback for low e (yrs) 8 Fairbanks, AK $537 (18%) Anchorage, AK $406 (23%) Duluth, MN $422 (24%) Solar Control Low e Glass Over Single Pane Wood Window Energy Cost Savings Simple Payback for Total Product (yrs) Simple Payback for low e (yrs) 6 A Burlington, VT $664 (25%) A Minneapolis, MN $346 (25%) A Boston, MA $470 (28%) A Rochester, NY $510 (24%) A Pittsburgh, PA $375 (24%) A Chicago, IL $299 (25%) B Denver, CO $239 (24%) B Boise, ID $249 (25%) A New York, NY $418 (25%) A Washington, DC $352 (24%) $330 (22%) A Raleigh, NC $267 (20%) $255 (19%) A Kansas City, MO $368 (27%) $350 (26%) C Seattle, WA $294 (29%) A Atlanta, GA $263 (26%) $255 (26%) A Dallas, TX: Furnace Heat pump $212 (25%) $174 (21%) $229 (26%) $198 (24%) 2 A Jacksonville, FL $135 (18%) $168 (23%) A Houston, TX: Furnace Heat pump $166 (22%) $143 (19%) $197 (26%) $178 (24%) 2 B Phoenix, AZ $196 (18%) $247 (23%) A Miami, FL $117 (13%) $183 (21%) Notes: Payback will be shorter for leakier primary windows, higher fuel costs, or propane and electrical resistance heating. Gray cells not evaluated

27 Table 3.5. HVAC Source Energy Savings for Low-e Storm Windows and Panels as Calculated by RESFEN Larger, Newer, Two-Story Home Climate Zone City, State HVAC Source Energy Savings of Low-e Storm Windows Used Over Different Primary Window Types in Larger Newer 2-story Home Single-Pane, Wood- Framed Window Standard Low-e Glass Double- Pane, Metal- Framed Window Double-Pane, Wood- Framed Window 8 Fairbanks, AK 40 43% 28 30% 21 24% 7 Anchorage, AK 45 48% 32 35% 24 28% 7 Duluth, MN 46 50% 32 35% 24 28% Single- Pane, Wood- Framed Window Solar Control Low-e Glass Double- Pane, Metal- Framed Window Double-Pane, Wood- Framed Window 6 A Burlington, VT 44 48% 31 34% 23 27% 6 A Minneapolis, MN 44 47% 30 33% 23 26% 5 A Boston, MA 45 49% 31 35% 24 27% 5 A Rochester, NY 43 46% 30 32% 23 26% 5 A Pittsburgh, PA 43 46% 30 33% 23 26% 5 A Chicago, IL 44 47% 30 33% 23 26% 5 B Denver, CO 47 50% 34 37% 26 30% 5 B Boise, ID 45 48% 33 35% 25 28% 4 A New York, NY 43 46% 29 32% 22 25% 4 A Washington, DC 40 43% 28 31% 21 24% 39% 27% 20% 4 A Raleigh, NC 37 39% 25 27% 20 21% 36% 25% 20% 4 A Kansas City, MO 39 42% 27 29% 21 23% 39% 26% 20% 4 C Seattle, WA 49 53% 36 39% 28 32% 3 A Atlanta, GA 36 38% 25 27% 20 21% 37% 25% 20% 3 A Dallas, TX: Furnace Heat pump 34 35% 31% 23 24% 20 21% 19% 18 17% 36% 35% 25% 25% 2 A Jacksonville, FL 27 26% 17% 16 14% 33% 24% 23% 2 A Houston, TX: Furnace Heat pump 30 29% 27 26% 19% 17% 17 16% 17 15% 34% 33% 24% 24% 21% 22% 23% 23% 2 B Phoenix, AZ 25 24% 18% 16 14% 32% 25% 23% 1 A Miami, FL 17 14% 9 8% 12 9% 27% 19% 22% Notes: Range of savings indicates exterior and interior low-e panels (first and second numbers, respectively). Solar control low-e panels were modeled for exterior application only. Installation over metal-framed window assumes thermally broken mounting. Gray cells not evaluated. 17

28 Table 3.6. HVAC Energy Cost Savings of Exterior Low-e Storm Windows over Single-Pane Wood Windows as Calculated by RESFEN Larger, Newer, Two-Story Home Climate Zone Location HVAC Energy Cost Savings of Exterior Low-e Storm Windows in Larger, Newer, Two-Story Home Standard Low-e Glass Over Single-Pane Wood Window Energy Cost Savings Simple Payback for Total Product (yrs) Simple Payback for low-e (yrs) 8 Fairbanks, AK $824 (40%) Anchorage, AK $619 (45%) Duluth, MN $645 (46%) Solar Control Low-e Glass Over Single-Pane Wood Window Energy Cost Savings Simple Payback for Total Product (yrs) Simple Payback for low-e (yrs) 6 A Burlington, VT $986 (45%) A Minneapolis, MN $516 (43%) A Boston, MA $688 (45%) A Rochester, NY $749 (42%) A Pittsburgh, PA $547 (43%) A Chicago, IL $442 (43%) B Denver, CO $340 (45%) B Boise, ID $363 (45%) A New York, NY $615 (42%) A Washington, DC $508 (41%) $488 (39%) A Raleigh, NC $369 (38%) $362 (37%) A Kansas City, MO $529 (41%) $512 (39%) C Seattle, WA $425 (50%) A Atlanta, GA $410 (39%) $394 (38%) A Dallas, TX: Furnace Heat pump $317 (34%) $260 (31%) $334 (36%) $292 (35%) 2 A Jacksonville, FL $190 (27%) $234 (33%) A Houston, TX: Furnace Heat pump $240 (30%) $208 (27%) $278 (35%) $254 (33%) 2 B Phoenix, AZ $254 (26%) $319 (32%) A Miami, FL $149 (17%) $234 (27%) Notes: Payback will be shorter for leakier primary windows, higher fuel costs, or propane and electrical resistance heating. Gray cells not evaluated

29 Table 3.7. Overall Average Savings and Payback Period by Climate Zone. Results in climate zones 1 3 are for solar control low e. Source Energy Savings Simple Payback for Total Product (years) Incremental Simple Payback for Low-e (years) Over Wood Frame, Single Pane Over Wood Frame, Single Pane Over Wood Frame, Single Pane Zone Average Min Max Average Min Max Average Min Max 8 30% 18% 43% % 23% 50% % 25% 48% % 24% 50% % 19% 53% % 24% 38% % 23% 34% % 21% 27% Source Energy Savings Simple Payback for Total Product (years) Incremental Simple Payback for Low-e (years) Over Metal Frame, Double Pane Over Metal Frame, Double Pane Over Metal Frame, Double Pane Zone Average Min Max Average Min Max Average Min Max 8 21% 12% 30% % 15% 35% % 16% 34% % 16% 37% % 13% 39% % 16% 27% % 16% 25% % 15% 19% Source Energy Savings Simple Payback for Total Product (years) Incremental Simple Payback for Low-e (years) Over Wood Frame, Double Pane Over Wood Frame, Double Pane Over Wood Frame, Double Pane Zone Average Min Max Average Min Max Average Min Max 8 16% 8% 24% % 10% 28% % 11% 27% % 11% 30% % 9% 32% % 12% 22% % 15% 23% % 17% 22%